Arrhythmias, particularly sinus tachycardia, are often seen after open heart surgery or in the course of an acute myocardial infarct. Pharmacologic agents are often ineffective to control these arrhytmias, or may be contra-indicated. Carotid sinus pressure and various types of pacing such as rapid atrial pacing, paired or coupled pacing, and timed extra systoles, are often ineffective, especially in febrile states. In the O.R. during open heart surgery we have seen that heart rate is easily and quickly controlled by slight local temperature changes in the SA node. We have evaluated this effect intially in an animal model, using open chest preparation. Small cooling probes were designed to operate in conjunction with miniature thermocouples to cool and monitor the SA node. Initial data demonstrates the feasibility for varying SA node temperature in order to control heart rate within a clinically useful range. It is expected that complete data on this phenomena will lead to a simple technique for controlling arrhythmias which will have wide clinical use.